Organic electroluminescent devices (hereinafter referred to as organic EL devices) are new self-emitting devices of great prospects. The organic EL devices have a stacked structure in which a carrier transporting layer (an electron or hole transporting layer) and a light emitting layer are sandwiched between a hole injecting electrode and an electron injecting electrode.
An electrode material having a large work function, such as gold or ITO (indium-tin oxide), is used to form the hole injecting electrode, and an electrode material having a small work function, such as Mg (magnesium) or Li (lithium), is used to form the electron injecting electrode.
The hole transporting layer, light emitting layer, and electron transporting layer are formed of organic materials. A material having p-type semiconductor properties is used to form the hole transporting layer and a material having n-type semiconductor properties is used to form the electron transporting layer. The light emitting layer is formed of a fluorescent or phosphorescent organic material which, too, has a carrier transporting property, as an electron or hole transporting property.
The hole injecting electrode, hole transporting layer, light emitting layer, electron transporting layer, and electron injecting electrode are stacked in this order to form an organic EL device.
The functional layers, i.e. the hole transporting layer, electron transporting layer, and light emitting layer, may each be composed of a plurality of layers, or may be omitted, depending on the organic materials used.
For example, Chihaya Adachi et al., Appl. Phys. Lett., Vol. 55, pp. 1489-1491 (1989) discloses a device structure that has only two organic layers, light emitting and electron transporting layers, between the hole injecting electrode and electron injecting electrode. In this device, the light emitting layer made of a luminescent material called NSD has a good hole transporting property, so that the light emitting layer can act also as a hole transporting layer.
C. W. Tang et al., Appl. Phys. Lett., Vol. 51, pp. 913-915 (1987) discloses a device structure that has two organic layers: hole transporting and light emitting layers. In this case, tris(8-hydroxyquinolinato)aluminum (hereinafter referred to as Alq) in the light emitting layer performs two functions: light emitting and electron transporting functions.
S. A. VanSlyke et al., Appl. Phys. Lett., Vol. 69, pp. 2160-2162 (1996) discloses a device structure that has three organic layers: a hole injecting layer, hole transporting layer, and light emitting layer. In this case, the hole injecting layer, made of copper phthalocyanine, serves like a hole transporting layer; thus the entire device includes two hole transporting layers.
In this way, the stacked structure of the electron transporting layer, hole transporting layer and light emitting layer can be freely designed, depending on the organic materials used.
Organic EL devices can be used to obtain visible light from blue to red, by properly selecting organic material of the light emitting layer. Therefore a full-color display can be implemented by using monochromatic organic EL devices individually emitting red, green or blue light, i.e. the three primary colors of light (RGB).
Among red, green and blue lights obtained with organic EL devices, green light and blue light are stable. On the other hand, it is difficult to obtain light with high luminance and high luminous efficiency in the range from red to orange. Therefore developing full-color displays requires red-emitting organic EL devices with good color purity, high luminous efficiency, and high luminance.
JP2000-164362, A suggests a method in which rubrene having the molecular structure represented by Formula (15) below is used as a light-emission assisting dopant. While this method offers improved red color purity, it fails to provide sufficient luminous efficiency and sufficient luminance.
